1. Field of the Invention
The invention relates to a device for superposing optical signals of different wavelengths, the device being provided with an input free beam coupler and an output free beam coupler the free beam sections of which are coupled to each other by phase shift light guides of a light guide grid, with a first input light guide unit connected to the input free beam coupler and with at least one further input light guide unit whereby at least one wavelength may be conducted in each input light guide unit, and with an output light guide unit provided with a plurality of output light guides, the output light guides being connected to the output free beam coupler and arranged such that each of the output light guides may be energized by at least two optical signals of different wavelengths.
2. The Prior Art
Such a device is known from U.S. Pat. No. 5,440,416. That prior art device is provided with an input free beam coupler and an output free beam coupler the free beam sections of which are coupled to each other by a light guide grid. Furthermore, the prior art device is provided with two input light guide units each one of which is provided with an input light guide. Optical signals of different wavelengths may be fed into the input light guide of one input light guide unit. This input light guide is connected to the input free beam coupler. The input light guide of the other input light guide unit is connected to the side of the output free beam coupler which is connected to the light guide grid. In addition, output light guides of an output light guide unit are connected to the output free beam coupler, the output light guides being arranged such that each of them may be energized by a signal from the input light guide connected to the input free beam coupler and by a signal from the input light guide connected to the output free beam coupler.
Because of the imaging function of the so-called phased array spectograph formed by the input free beam coupler, the light guide grid and the output free beam coupler, the optical signals of different wavelengths propagating in the input light guide of the input light guide unit connected to the input free beam coupler are each conducted to associated output light guides. The optical signal propagating in the input light guide of the input light guide unit connected to the output free beam coupler is spreading as a spherical wave into the free beam section of the output free beam coupler and impinges upon the output side of the output free beam coupler to which the output light guides are connected.
While it is possible with the prior art device to carry out the function of a multiplexer by feeding the wavelengths propagating in the two input light guide units to associated output light guides, it suffers from the disadvantage of resultant relatively high losses.
An optical multiplexer is also known from EP 528,652 which is not, however, provided with any further input light guide unit. This publication describes the periodic transfer characteristic which makes possible the occurrence in an output waveguide of signals of wavelength period xcex1xcex from an input waveguide (i.e. in addition to wavelength xcex1 there occurs the next periodically possible wavelength xcex2=xcex1+xcex94xcex and, further, xcex3=xcex1+2*c16536D xcex94xcex. . . etc.) This constitutes a specific embodiment of a spectograph in planar waveguide technology.
It is an object of the invention to provide a device of the kind referred to hereinabove which is characterized by low attenuation losses.
The object is accomplished by the at least one other input light guide unit converging into the free beam section of the input free beam coupler and by converging sections of the input light guide units being arranged and dimensioned according to the equation.                     Θ        IN                  (          i          )                    =                        m                                    n              s                        ⁢            d                          ⁢                  {                                    λ                              (                i                )                                      -                                                                                λ                    c                                    [                                      1                    +                                                                  1                                                  n                          c                                                                    ⁢                                              xe2x80x83                                            ⁢                                                                        ⅆ                                                      n                            c                                                                                                    ⅆ                          λ                                                                                                      "RightBracketingBar"                                                  λ                  =                                      λ                    c                                                              ⁢                              (                                                      λ                                          (                      i                      )                                                        -                                      λ                    c                                                  )                                              ]                      }    -      Θ    OUT          (      i      )      
wherein xcex(i) is the wavelength conducted in an input light guide of the input light guide units; xcexc is the central wavelength of the device; m is the order of refraction of the light guide grid; nc is the effective index of refraction of the phase shift light guide; ns is the effective refractive index of the free beam sections; d is the grid constant or pitch of the phase shift light guides at the transition to the output free beam section of the output free beam coupler; xe2x8ax96(i)IN is the incoupling angle of the ith input light guide relative to the input axis of symmetry and xe2x8ax96(i)OUT is the outcoupling angle relative to the output axis of symmetry of the output free beam section, such that each output light guide may be charged with at least two optical signals of different wavelengths which are conducted in at least two input light guide units.
The central wavelength xcexc of the device is derived from the mean or average between the broadcast wavelength or the mean wavelength of the specific wavelengths.
By connecting the input light guide units to the input free beam coupler and by arranging and dimensioning the output sections of the input light guide units relative to the output light guides such that each output light guide may be charged with at least two optical signals of different wavelengths, the optical signals of the input light guide sections are fed at low attenuation losses and in a controlled manner to the output light guides by way of the imaging function of the so-called phased array spectograph formed by the input free beam coupler, the light guide grid and the output free beam coupler. By separating the power division which takes place entirely within the input free beam coupler, from the imaging controlled superposition in the output free beam coupler, optimizing measures may be separately carried out effectively at the input free beam coupler and the input light guide units connected thereto and the output free beam coupler and its connected output light guide unit.
In one embodiment of the invention at least a first input light guide unit is provided with a plurality of input light guides connected to the input free beam coupler. In a related variant, optical signals of different wavelengths grouped around a center wavelength may be conducted in the individual input light guides of the first input light guide unit, the input light guides being arranged and dimensioned relative to the output light guides such that each output light guide is provided with an associated input light guide of the first input light guide unit.
In another variant, there is provided a second input light guide unit provided with individual input light guides which may be charged with an optical signal of a single wavelength, the input light guides being arranged and dimensioned relative to the output light guides such that each output light guide is associated with an input light guide of the second input light guide unit.
In an advantageous embodiment, there are provided two input light guides provided with input light guides as in the preceding embodiments so that each output light guide is associated with exactly one input light guide of the first input light guide unit and one input light guide of the second light guide unit. This results in a particularly low systems attenuation at a controlled multiplex function.
In a further embodiment, at least one input light guide unit is provided with an input light guide which preferably flares out hyperbolically in the direction of the input free beam coupler. Into this single flaring input free beam coupler an optical signal of a single wavelength may be fed which signal is to be distributed to the output light guides. While such a structure leads to somewhat higher attenuation losses, it permits coupling-in so-called broadcast wavelengths which fluctuate relatively strongly, for the flaring of the input light guide ensures that even at fluctuating wavelengths the output light guides are charged with the corresponding optical signal.